An automotive suspension system essentially consists of a suspension arm, a spring and a damper. The damper is placed in a wheel house typically in an upright orientation, and the upper end thereof is supported by the upper wall of the wheel house. Because the damper applies a vertical impulsive load on the wheel house as the automobile travels over an irregular road surface, if the rigidity of the wheel house is not adequate, the wheel house deforms, and this adversely affects the stability of the motion of the automobile. Therefore, it has been proposed to fit the outboard wall of the wheel house with a damper stiffener consisting of a frame member defining a closed cross section in cooperation with the outboard wall of the wheel house (Japanese patent laid open publication No. 07-309252).
Also, a rear wheel panel defining the floor of a trunk room is often fitted with a pair of floor frames attached to either side of the floor panel and a floor cross member each consisting of a frame member so that the trunk room floor may be provided with an adequate mechanical strength and rigidity that are required to withstand the load of rear seats and luggage (Japanese patent laid open publication No. 08-216927). The floor cross member and floor frames typically each consist of a channel member having a rectangular C-shaped cross section which is spot welded to the floor panel so as to jointly define a closed cross section. Each end of the floor cross member is spot welded to the inboard wall of the corresponding floor frame.
An automotive body, particularly a monocoque body is required to have an adequate torsional and bending rigidity for the automobile to have a desired motion stability and a high ride quality. However, the damper stiffener disclosed in Patent Document #1 may increase the rigidity of the wheel house, but is not effective in increasing the overall rigidity of the rear part of the body. The floor frames and the floor cross member disclosed in Patent Document #2 may be effective in increasing the rear floor panel, but are not very effective in increasing the overall rigidity of the rear part of the body. It was proposed to align the fore-and-aft positions of the damper stiffeners and the floor cross member with each other, but because the floor frames break the continuity between the closed cross section of each damper stiffener and the closed cross section of the floor cross member, the overall rigidity of the rear body part could not be effectively increased.